Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 38

Qinghai-Tibet Plateau, which was triggered by an earthquake (Ms 7.5) in 1955. Figure 1.32(b) shows a
landslide dam triggered by the Wenchuan earthquake (Ms 8.0) on the Zongqu Ravine in Maoxian, Sichuan
on May 12, 2008. The Zongqu Ravine was deeply incised and the slopes are very steep. Although the ravine
is not close to the epicenter but the landslide occurred because of bed incision makes the slopes unstable.
(a) (b)
Fig. 1.32 (a) A huge avalanche on the deeply incised Kangding River on the eastern margin of the Qinghai-Tibet
Plateau, which was triggered by an earthquake (Ms 7.5) in 1955; (b) A landslide triggered by the Wenchuan earthquake
created a 70 m high landslide dam on the Zongqu Ravine in Maoxian, Sichuan on May 12, 2008
The eastern margin of the Qinghai-Tibet Plateau is one of the world's broadest and most dramatic
transient landscapes that has been deeply incised by major rivers and their tributaries (Clark et al., 2006).
Major rivers start at high elevations over 4,000 m, where they are slightly incised into the relict landscape,
and transition into rapidly incising, high-relief, dissected gorges with steep hillslopes (Ouimet et al., 2007).
Hillslopes, following incision, display zones of adjustment with steepest values in the lowermost reaches
of individual basins. These observations highlight the transient response of rivers to rapid incision on
trunk streams as waves of landscape adjustment propagate upstream and up hillslopes.
The Dadu River and Yalong River are major tributaries of the Yangtze River, both over 1,000 km in
length. The Dadu and Yalong rivers have each experienced catastrophic landslide damming events within
the past 250 years that were triggered by large earthquakes (>Ms 6.0) (Tianchi, 1990; Dai et al., 2005).
Accounts from the example on the Dadu River indicate as many as 100,000 deaths were caused by the
downstream flooding associated with initial dam failure, making it one of the most disastrous events ever
resulting from a landslide dam breach (Dai et al., 2005). Recent catastrophic events such as these have
also been documented in the Min River gorge to the east of the Dadu and in the main Yangtze River gorge
to the west of the Yalong, indicating that landslide damming is a widespread and ongoing phenomenon
in all river gorges on the eastern margin (Tianchi, 1990; Chai et al. 2000). The dynamic coupling between
river incision and landslide dam can significantly influence the evolution of fluvial landscapes. An important
feedback exists within rapidly incising landscapes, where hillslope erosion, following incision, slows or
stops river incision by covering the bed with larger volumes of sediment or coarser grain sizes of sediment
than annual floods can transport downstream.